The invention relates to an input circuit for high-frequency amplifiers having four transistors of the same conductivity type.
Input circuits for intermediate and high-frequency amplifiers sometimes have an unsymmetrical input and a symmetrical output. As the input resistance, values between 50 .OMEGA. and 300 .OMEGA. are mostly required, as in use in terminating resistors for cables and filters. Further amplifier stages or selection means are mostly connected to the output.
Applications for such input circuits are, for example, cable output amplifiers for wide-band transmission channels, antenna amplifiers and intermediate-frequency amplifiers with low input reistance for adjustment to a previous filter. A further advantageous embodiment is initial amplification of the signals from light-sensitive diodes. In a conventional solution of the problem, bipolar transistors in common-base connection or "intermediate-base connection" (transistor circuit in which the center tap of a coil, connected between base and emitter, is earthed) are used, and matching transformers frequently employed.
A solution more suited to integration is known from DE-PS No. 29 46 952. In the known circuit, in a differential stage comprising two bipolar transistors and a current source, a third transistor is connected to the interconnected emitters of the two transistors, namely by its emitter to the negative connection point of the current supply or earth, by its base to the emitters of the two transistors, and by its collector to a load resistor running to the positive connection point of the current supply. Furthermore, two resistors R.sub.a, R.sub.b are connected to the collector of the third transistor and each run to an input (A, B) of the differential stage, i.e. to the base of the first and second transistor respectively. In a differential stage of this type, the potential of the inputs is fixed at a value of about twice the forward voltage above earth potential. If an input signal U.sub.e is applied to input A, the effect mechanism of this circuit results in the generation of a reversed signal--U.sub.e at input B, and the differential stage behaves as if symmetrically driven with 2 U.sub.e. Half the value of resistance R.sub.a is effective here as the input resistance. However, this only applies for frequencies which are not too high. If the transistor parameters become frequency-dependent, the input resistance will also become so. The known circuit also has the drawback as regards frequency behaviour that it has an uncompensated feedback of the differential stage outputs via the collector/base capacitance to the inputs. A further drawback of the known circuit is also the effect of the noise contribution by the third transistor. Although this contribution disappears when both inputs are wired in the same way, this is not usually possible. The noise of the third transistor contributes to the unavoidable noise of the differential stage.